A wind turbine known in the art comprises a tapered wind turbine tower and a wind turbine nacelle positioned on top of the tower. A wind turbine rotor with a number of wind turbine blades is connected to the nacelle through a low speed shaft, which extends out of the nacelle front as illustrated on FIG. 1.
Oscillations and vibrations of the wind turbine blades are undesirable in that, they can in worse case damage the blades. In particular edge-wise oscillations, which are oscillations along the cord between the trailing edge and the leading edge of the blade, can damage the blade, as it have little damping towards this mode of oscillations.
Furthermore, edgewise oscillations are particularly harmful, in that they among other things can cause cracks at the root of the blade or along the trailing edge. In known cases such oscillations has caused the blade to fail to such degree, that the blade has disintegrated from the turbine.
Both stall and pitch controlled wind turbine are in risk of being damaged by edge-wise oscillations. The stall controlled turbine is mostly seeing this problem when operating in high winds beyond the stall point. The pitch regulated turbine is mostly seeing this problem when parked in high wind with the rotor locked.
To avoid oscillations of the blade it is known to provide the blades with different forms of mechanical dampers, most often based on the principle of a spring mounted mass combined with a damping device or they can be provided with different kinds of liquid dampers.
An example of a liquid damper is disclosed in WO 99/32789, where the tips of the blades are provided with a tuned liquid damper system. A liquid flows freely in a number of cambers placed as close to the tip of the blade as possible. The chambers have a specific length, which is adapted to the natural edgewise frequency of the specific blade type. Even though these kinds of frequency specific dampers weigh less than traditional multi-frequency dampers, they still have the disadvantage of adding considerable weight to the tip of the blade, where weight is least desired. The damping capacity is proportional with the width of the damper and the frequency.
As modern wind turbines get bigger both in output and in size, the length and the size of the blade also increase. As the blade becomes bigger and heavier their natural edgewise frequency becomes lower—down to a few Hz or even bellow one Hz, and the blade therefore becomes easier to excite by the wind. As the natural edgewise frequency gets lower, the mass of a mechanical damper, a liquid damper or a tuned liquid damper has to be increased, which leads to an increase in size.
The width of the blade decreases towards the tip, and when the dampers get longer and wider, the space inside the blade at the tip becomes too small to contain the damper. The damper has to be moved further away from the tip, and the further from the tip it is moved, the bigger and heavier it has to be. This is of cause disadvantageous, in that the heavier the blades are, the more load is induced to other components of the wind turbine. This requires stronger components which most often are more expensive.
Another disadvantage in traditional blade dampers is, that dampers placed close to the tip of the blade will also inevitably interfere with the load-carrying structure of the blade, hereby potentially compromising the structural integrity of the blade.
An object of the invention is to provide for a wind turbine blade comprising an oscillation damper without the mentioned disadvantages.
Especially it is an object of the invention to provide technique for damping oscillations of a wind turbine blade, which are efficient even in large and long wind turbine blades.